Fire protection in areas with high concentrations of instrument, communication and power transmission wires and cables is very important, especially in such installations as large buildings and power plants. Such wires and cables are constructed with electrically insulating coverings which are combustible and can provide a pathway by which fire might spread. Fire protection for such wires and cables is critical when power supply is necessary for the maintenance of controls, for example, in aircraft or factories.
Intumescent sheets for fire protection are disclosed in U.S. Pat. No. 4,273,879. These particular sheets have a significant content of organic materials (binder and char forming resin). During combustion, these organic materials can cause exothermic reactions increasing the temperature of the fire.
There are other fire protection materials which may be produced in a sheet form and which are designed to retard the spread of fire and heat by an endothermic reaction. See, for example, U.S. Pat. No. 4,315,075 describing a neoprene foam containing hydrated alumina.
Alumina trihydrate (Al.sub.2 O.sub.3..sub.3 H.sub.2 O) has 34.6% chemically combined water of hydration. This water is liberated beginning at about 230.degree. C. with complete dehydration at about 600.degree. C. This endothermic dehydration is known to have a cooling effect in compositions of which alumina trihydrate is a component. The water vapor given off also dilutes the combustible gases which may be present during a fire to help arrest combustion. Alumina trihydrate (ATH) is a known fire retardant filler in the plastics industry.
Inorganic materials in the form of fiber blankets have been used for the protection of conduits and cable trays. One such fiber blanket is prepared from alumino-silicate glass fibers and marketed under the trademark Kaowool by The Babcock & Wilcox Company. In order to have one hour fire protection, the alumino-silicate fiber blankets are wrapped around cable trays in a thickness of usually two inches or more. This blanket insulation has low thermal conductivity and, therefore, not only insulates from the effects of fire but also retains in the conduit the heat generated by the current carried through electrical cables under normal conditions. With the dissipation of normal heat generated by the line resistance thus retarded, the cables in trays or conduit must be derated. That is, the amount of current which they are rated to carry (ampacity or amp capacity) must be decreased.
It is desired to provide a composition which gives good fire protection to cable trays and conduits with a minimum reduction in the ampacity rating of the electrical cables in them.